1. Field of the Invention
The present invention relates to a pneumatic radial tire for four-wheeled vehicles and more specifically to thickness of blocks of a pneumatic radial tire suitable for use at high speed travel.
2. Description of the Prior Art
In conventional pneumatic radial tires, the tread is always formed with a plurality of circumferential grooves arranged at regular intervals, a plurality of transverse grooves intersecting the circumferential grooves and arranged at regular intervals along the tire circumferential direction, and blocks each partitioned by the above circumferential and transverse grooves. Further, the thickness of the blocks is always uniform extending from a center-side circumferential groove to an sidewall-side circumferential groove. That is, above-mentioned .uniform tread thickness is applied to all the prior-art tires, irrespective of size, usage, or aspect ratio of tires.
In the conventional radial tires, no problem will arise when the tire is used at travel speed less than 200 km/h. However, when tire travel speed exceeds 250 km/h, the tire blocks are pushed or projected into outwardly convex shape by centrifugal force due to high-speed tire rotation (this projection is the maximum at each block center). As a result, the tire presents a complicated tread contour having various outer block surfaces with different radii of curvatures. Once the originally-uniform contours or outer surfaces of the blocks deform during travel, the contact pressure between the tire and the ground increases extraordinarily at the centers of blocks (at which the tread projection reaches its maximum) as compared with that at the other areas of blocks. The fact that tire contact pressure is high indicates that compressive deformation of tire rubber is great. Further, after the tire block has passed through a contact time point with the ground, since the compressive deformation restores, the above-mentioned compressive deformation and restoration are repeated whenever the tread or block passes through the contact time points with ground or for each tire revolution. In the case of the passenger cars, since the tire rotates 40 revolution per second at 300 km/h, the above-mentioned tire compressive deformation and restoration provide very severe conditions for the tire at the center of each block. The above repeated tire compressive deformations cause the tire to be heated in particular at the centers of blocks (because the centrifugal force is applied to the mass of the block and further the heat radiation effect is low as compared with the other areas of blocks). This results in a problem in that tire rubber changes in quality (into sponge state) due to heat and therefore the tire blocks are easily blown out.